This invention relates generally to electrical fuses and, in particular, to electrical fuses having a fusible link extending between a pair of terminal portions.
Known electrical fuses have taken many forms and generally comprise fuses having a fusible link extending between a pair of terminal portions. The fusible link may be provided either with notches cut in one or more sides of the fusible portion or with holes formed therethrough to create narrower and therefore weaker portions within the fusible portion.
In at least some types of fuses, the holes within the fusible links are filled with a material having a lower melting point than the parent metal of the fusing portion. As the fusible link is heated during an electrical overload, the lower melting-point material diffuses into the parent metal, thereby raising the electrical resistance of the fusible link and further increasing the electrical load on the narrow and weaker portions of the fusible link. When the load reaches a sufficient magnitude, the fusible link fails and the electrical connection is no longer maintained. The presence of the lower melting point material modifies operational characteristics of the fusible link such that the highest current it will carry indefinitely without failing or melting is reduced while its behavior at higher currents is substantially unaffected. This phenomenon is sometimes referred to as a xe2x80x9cMetcalf effectxe2x80x9d or xe2x80x9cM-effectxe2x80x9d.
One disadvantage of such a fuse link construction including holes in the fusible link is that it provides two weak points in parallel with each other, i.e., one weak point on each side of the hole in the fusible link. To achieve consistent fusing performance between individual fuses, the two parallel weak points should be very accurately matched. Slight differences between the cross-sectional areas of the two weak points will lead to an undesirable imbalance in the current flowing through each of the weak points, which further results in a temperature imbalance between the two weak points. Since, for a given fuse current rating the cross-sectional area of each of the parallel weak points will account for roughly one half that of the fuse current rating, accurate and repeatable fuse element manufacture is difficult, particularly for fuses of low current rating.
In an exemplary embodiment of the invention an electrical fuse includes a pair of terminal portions and a fusible link extending between the pair of terminal portions. The fusible link includes a fusing portion and a modifying portion in contact with the fusing portion. The modifying portion is formed of a material having a lower melting point than the fusing portion, and the fusible link includes a hole extending therethrough and defining an open-sided receptacle. A side of the open-sided receptacle forms one side of the fusing portion, and the modifying portion is disposed within the substantially open-sided receptacle.
A method of manufacturing the electrical fuse includes the steps of forming a hole extending through the fusible link to define an open sided receptacle, forming the modifying portion within the substantially open-sided receptacle by disposing a body of lower melting point material therein, and melting and reflowing the body of lower melting point material into intimate contact with the side of the open-sided receptacle.
A fuse having a single reliable fusible portion is therefore provided that is particularly advantageous for low current rating fuse applications wherein conventional fuses have been disadvantaged. Using relatively simply construction techniques, accurate and repeatable low current fuses may be produced.